A common failure mode for X-ray tubes, particularly grid controlled tubes, is a short circuit between the cathode filament and the cathode cup to which the filament is mounted. The filament is energized to emit electrons and a potential of several kilovolts is required between the filament and the cathode cup to control the electron beam. Many measures are taken in the design and manufacture of an X-ray tube to prevent tube filaments from shorting since this is a catastrophic failure, requiring the replacement of the X-ray tube. Under certain fault of user conditions, however, the filament may become overheated in localized regions. This situation, in concert with the high electric field can cause the filament to bow into contact with and short circuit the cathode cup, destroying the x-ray tube.
Commonly assigned U.S. patent application Ser. No. 752,311 to Furbee et al. entitled "X-Ray Tube Electron Beam Switching and Biasing Method and Apparatus", now U.S. Pat. No. 4,685,118, discloses certain manufacturing measures that can be taken to reduce the occurrence of X-ray tube filament shorting. The disclosure of this patent is incorporated herein by refernce. Even these measures, however do not entirely eliminate filament shorting.